Add code for verifying homeservers' signatures on API requests

lib/matrix_server.ex

@@ -1,4 +1,6 @@
 defmodule MatrixServer do
+  alias MatrixServer.OrderedMap
+
   def get_mxid(localpart) when is_binary(localpart) do
     "@#{localpart}:#{server_name()}"
   end
@@ -54,4 +56,28 @@ defmodule MatrixServer do
     |> Base.encode64()
     |> String.trim_trailing("=")
   end
+
+  # Decode (possibly unpadded) base64.
+  def decode_base64(data) when is_binary(data) do
+    rem = rem(String.length(data), 4)
+    padded_data = if rem > 0, do: data <> String.duplicate("=", 4 - rem), else: data
+    Base.decode64(padded_data)
+  end
+
+  def encode_canonical_json(object) do
+    object
+    |> Map.drop([:signatures, :unsigned])
+    |> OrderedMap.from_map()
+    |> Jason.encode()
+  end
+
+  # https://stackoverflow.com/questions/41523762/41671211
+  def to_serializable_map(struct) do
+    association_fields = struct.__struct__.__schema__(:associations)
+    waste_fields = association_fields ++ [:__meta__]
+
+    struct
+    |> Map.from_struct()
+    |> Map.drop(waste_fields)
+  end
 end

lib/matrix_server/schema/event.ex

@@ -5,7 +5,6 @@ defmodule MatrixServer.Event do
 
   alias MatrixServer.{Repo, Room, Event, Account, OrderedMap, SigningServer}
 
-  @schema_meta_fields [:__meta__]
   @primary_key {:event_id, :string, []}
   schema "events" do
     field :type, :string
@@ -287,7 +286,7 @@ defmodule MatrixServer.Event do
   defp calculate_content_hash(event) do
     result =
       event
-      |> to_map()
+      |> MatrixServer.to_serializable_map()
       |> Map.drop([:unsigned, :signature, :hashes])
       |> OrderedMap.from_map()
       |> Jason.encode()
@@ -305,7 +304,7 @@ defmodule MatrixServer.Event do
   defp redact(%Event{type: type, content: content} = event) do
     redacted_event =
       event
-      |> to_map()
+      |> MatrixServer.to_serializable_map()
       |> Map.take([
         :event_id,
         :type,
@@ -347,14 +346,4 @@ defmodule MatrixServer.Event do
         "users",
         "users_default"
       ])
-
-  # https://stackoverflow.com/questions/41523762/41671211
-  def to_map(event) do
-    association_fields = event.__struct__.__schema__(:associations)
-    waste_fields = association_fields ++ @schema_meta_fields
-
-    event
-    |> Map.from_struct()
-    |> Map.drop(waste_fields)
-  end
 end

lib/matrix_server/signing_server.ex

@@ -1,8 +1,6 @@
 defmodule MatrixServer.SigningServer do
   use GenServer
 
-  alias MatrixServer.OrderedMap
-
   # TODO: only support one signing key for now.
   @signing_key_id "ed25519:1"
 
@@ -16,8 +14,8 @@ defmodule MatrixServer.SigningServer do
     GenServer.call(__MODULE__, {:sign_object, object})
   end
 
-  def get_signing_keys do
-    GenServer.call(__MODULE__, :get_signing_keys)
+  def get_signing_keys(encoded \\ false) do
+    GenServer.call(__MODULE__, {:get_signing_keys, encoded})
   end
 
   ## Implementation
@@ -35,12 +33,7 @@ defmodule MatrixServer.SigningServer do
         _from,
         %{private_key: private_key} = state
       ) do
-    ordered_map =
-      object
-      |> Map.drop([:signatures, :unsigned])
-      |> OrderedMap.from_map()
-
-    case Jason.encode(ordered_map) do
+    case MatrixServer.encode_canonical_json(object) do
       {:ok, json} ->
         signature =
           json
@@ -62,10 +55,10 @@ defmodule MatrixServer.SigningServer do
     end
   end
 
-  def handle_call(:get_signing_keys, _from, %{public_key: public_key} = state) do
-    encoded_public_key = MatrixServer.encode_unpadded_base64(public_key)
+  def handle_call({:get_signing_keys, encoded}, _from, %{public_key: public_key} = state) do
+    result = if encoded, do: MatrixServer.encode_unpadded_base64(public_key), else: public_key
 
-    {:reply, [{@signing_key_id, encoded_public_key}], state}
+    {:reply, [{@signing_key_id, result}], state}
   end
 
   # TODO: not sure if there is a better way to do this...

lib/matrix_server_web/authenticate_server.ex

@@ -0,0 +1,102 @@
+defmodule MatrixServerWeb.AuthenticateServer do
+  import Ecto.Changeset
+
+  alias MatrixServer.SigningServer
+  alias Ecto.Changeset
+
+  @auth_header_regex ~r/^X-Matrix origin=(?<origin>.*),key="(?<key>.*)",sig="(?<sig>.*)"$/
+
+  defmodule SignedJSON do
+    use Ecto.Schema
+
+    import Ecto.Changeset
+
+    @primary_key false
+    embedded_schema do
+      field :method, :string
+      field :uri, :string
+      field :origin, :string
+      field :destination, :string
+      field :content, :map
+      field :signatures, :map
+    end
+
+    def changeset(params) do
+      %__MODULE__{}
+      |> cast(params, [:method, :uri, :origin, :destination, :content, :signatures])
+      |> validate_required([:method, :uri, :origin, :destination])
+    end
+  end
+
+  def authenticated?(%Plug.Conn{body_params: params}) do
+    with %Changeset{valid?: true} = cs <- SignedJSON.changeset(params),
+         input <- apply_changes(cs) do
+      verify_signature(input)
+    else
+      _ -> false
+    end
+  end
+
+  defp verify_signature(%SignedJSON{signatures: signatures, origin: origin} = input) do
+    if Map.has_key?(signatures, origin) do
+      # TODO: fetch actual signing keys from cache/key store.
+      signing_keys = SigningServer.get_signing_keys() |> Enum.into(%{})
+
+      found_signatures =
+        Enum.filter(signatures[origin], fn {key, _} ->
+          case String.split(key, ":", parts: 2) do
+            [algorithm, _] -> algorithm == "ed25519"
+            _ -> false
+          end
+        end)
+        |> Enum.map(fn {key_id, sig} ->
+          if Map.has_key?(signing_keys, key_id) do
+            {key_id, sig, signing_keys[key_id]}
+          end
+        end)
+        |> Enum.reject(&Kernel.is_nil/1)
+
+      with [{_, sig, signing_key} | _] <- found_signatures,
+           {:ok, raw_sig} <- MatrixServer.decode_base64(sig),
+           serializable_input <- MatrixServer.to_serializable_map(input),
+           {:ok, encoded_input} <- MatrixServer.encode_canonical_json(serializable_input) do
+        :enacl.sign_verify_detached(raw_sig, encoded_input, signing_key)
+      else
+        _ -> false
+      end
+    else
+      false
+    end
+  end
+
+  # TODO: Not actually needed?
+  def parse_authorization_headers(headers) do
+    headers
+    |> Enum.filter(&(elem(&1, 0) == "authorization"))
+    |> Enum.map(fn {_, auth_header} ->
+      Regex.named_captures(@auth_header_regex, auth_header)
+    end)
+    |> Enum.reject(&Kernel.is_nil/1)
+    |> Enum.reduce(%{}, fn %{"origin" => origin, "key" => key, "sig" => sig}, acc ->
+      Map.update(acc, origin, %{key => sig}, &Map.put(&1, key, sig))
+    end)
+  end
+
+  defmacro __using__(opts) do
+    except = Keyword.get(opts, :except) || []
+
+    quote do
+      def action(conn, _) do
+        action = action_name(conn)
+
+        if action not in unquote(except) and
+             not MatrixServerWeb.AuthenticateServer.authenticated?(conn) do
+          IO.puts("Not authorized!")
+          apply(__MODULE__, action, [conn, conn.params])
+        else
+          apply(__MODULE__, action, [conn, conn.params])
+        end
+      end
+    end
+  end
+end

lib/matrix_server_web/federation/key_controller.ex

@@ -9,7 +9,7 @@ defmodule MatrixServerWeb.Federation.KeyController do
 
   def get_signing_keys(conn, _params) do
     keys =
-      SigningServer.get_signing_keys()
+      SigningServer.get_signing_keys(true)
       |> Enum.into(%{}, fn {key_id, key} ->
         {key_id, %{"key" => key}}
       end)

lib/matrix_server_web/federation/test_controller.ex

@@ -0,0 +1,10 @@
+defmodule MatrixServerWeb.Federation.TestController do
+  use MatrixServerWeb, :controller
+  use MatrixServerWeb.AuthenticateServer
+
+  def test(conn, _params) do
+    conn
+    |> put_status(200)
+    |> json(%{})
+  end
+end

lib/matrix_server_web/federation_client.ex

@@ -7,17 +7,41 @@ defmodule MatrixServerWeb.FederationClient do
   # TODO: Maybe create database-backed homeserver struct to pass to client function.
 
   @middleware [
-    {Tesla.Middleware.Headers, [{"Content-Type", "application/json"}]},
     Tesla.Middleware.JSON
   ]
 
   @adapter {Tesla.Adapter.Finch, name: MatrixServerWeb.HTTPClient}
 
   def client(server_name) do
-    Tesla.client([{Tesla.Middleware.BaseUrl, server_name} | @middleware], @adapter)
+    Tesla.client([{Tesla.Middleware.BaseUrl, "http://" <> server_name} | @middleware], @adapter)
   end
 
   def get_signing_keys(client) do
     Tesla.get(client, RouteHelpers.key_path(Endpoint, :get_signing_keys))
   end
+
+  def test_server_auth(client) do
+    origin = "localhost:4001"
+    destination = "localhost:4000"
+    path = RouteHelpers.test_path(Endpoint, :test)
+
+    params = %{
+      method: "POST",
+      uri: path,
+      origin: origin,
+      destination: destination,
+      content: %{"hi" => "hello"}
+    }
+
+    {:ok, signed_object} = MatrixServer.SigningServer.sign_object(params)
+    auth_headers = create_signature_authorization_headers(signed_object, origin)
+
+    Tesla.post(client, path, signed_object, headers: auth_headers)
+  end
+
+  defp create_signature_authorization_headers(%{signatures: signatures}, origin) do
+    Enum.map(signatures[origin], fn {key, sig} ->
+      {"Authorization", "X-Matrix origin=#{origin},key=\"#{key}\",sig=\"#{sig}\""}
+    end)
+  end
 end

lib/matrix_server_web/plug/authenticate_client.ex

@@ -1,4 +1,4 @@
-defmodule MatrixServerWeb.Plug.Authenticate do
+defmodule MatrixServerWeb.Plug.AuthenticateClient do
   import MatrixServerWeb.Plug.Error
   import Plug.Conn
 

lib/matrix_server_web/router.ex

@@ -1,7 +1,10 @@
 defmodule MatrixServerWeb.Router do
   use MatrixServerWeb, :router
 
-  alias MatrixServerWeb.Plug.Authenticate
+  alias MatrixServerWeb.Plug.AuthenticateClient
+
+  # TODO: might be able to handle malformed JSON with custom body reader:
+  # https://elixirforum.com/t/write-malformed-json-in-the-body-plug/30578/13
 
   pipeline :public do
     plug :accepts, ["json"]
@@ -9,12 +12,11 @@ defmodule MatrixServerWeb.Router do
 
   pipeline :authenticate_client do
     plug :accepts, ["json"]
-    plug Authenticate
+    plug AuthenticateClient
   end
 
   pipeline :authenticate_server do
     plug :accepts, ["json"]
-    # TODO: Add plug to verify peer.
   end
 
   scope "/_matrix", MatrixServerWeb do
@@ -52,7 +54,9 @@ defmodule MatrixServerWeb.Router do
   end
 
   scope "/_matrix", MatrixServerWeb.Federation do
+    pipe_through :authenticate_server
 
+    post "/test", TestController, :test
   end
 
   scope "/", MatrixServerWeb.Client do